Patients with rapid cycling bipolar disorder (RCBD) experience at least four episodes of mania, hypomania, and/or depression in a year. They suffer significant morbidity and are frequently resistant to conventional treatments. An unstable sleep/wake cycle is both characteristic of the illness and feeds back on the symptoms so as to exacerbate them. In this series of projects, we are exploring whether interventions designed to stabilize the sleep-wake and circadian systems have mood-stabilizing effects in patients with RCBD. In patients with RCBD, we have explored systematically the effects on mood of daily treatment with bright light, extended scotoperiod (dark), or exogenous melatonin, as well as the effects on mood of acute bright or dim light. We found that chronic treatment with midday bright light or extended scotoperiod may have mood-stabilizing effects; the former is being studied further in a controlled clinical trial. Neither acute bright or dim light, daily evening light, nor daily exogenous melatonin had a significant effect on mood in patients with RCBD. Melatonin was prescribed in doses of 5.0 or 10.0 mg each day, administered at 8:00 or 10:00 p.m. There was preliminary evidence that morning phototherapy might destabilize mood in patients with RCBD.